This invention relates to musical instruments and more specifically to digitally controlled electronic instruments and methods for generating musical sound.
Digitally controlled methods of generating musical sound operate by producing a sequence of digital numbers which are converted to electrical analog signals. The analog signals are amplified to produce musical sound through a conventional speaker. Musical instruments which employ digital control are constructed with a keyboard or other input device and with digital electronic circuits responsive to the keyboard. The electronic circuits digitally process signals in response to the keyboard and digitally generate oscillations which form the sound in the speaker. These digitally generated oscillations are distinguished from oscillations generated by analog oscillators and are distinguished from mechanically induced oscillations produced by conventional orchestral and other type instruments.
All musical sounds, whether of electronic or mechanical origin, can be described by Fourier spectra. The Fourier spectra describes musical sound in terms of its component frequencies which are represented as sinusoids. The whole musical sound is, therefore, a sum of the component frequencies, that is, a sum of sinusoids.
Under Fourier analysis, tones are classified as harmonic or inharmonic. A harmonic tone is periodic and can be represented by a sum of sinusoids having frequencies which are integral multiples of a fundamental frequency. The fundamental frequency is the pitch of the tone. Harmonic instruments of the orchestra include the strings, the brasses, and the woodwinds. An inharmonic tone is not periodic, although it often can be represented by a sum of sinusoids. The frequencies comprising an inharmonic tone, however, usually do not have any simple relationship. Inharmonic instruments do not normally have any pitch associated with them. Instruments in the orchestra that are inharmonic include the percussion instruments, such as the bass drum, the snare drum, the cymbal and others.
Electronically controlled musical instruments have relied upon forming selected Fourier spectra as a basis for producing musical sound. One known type of digital musical instrument employs a harmonic summation method of music generation. In the harmonic summation method, a tone is produced by adding (or subtracting) a large number of amplitude-scaled sinusoids of different frequencies. The harmonic summation method, therefore, requires a large number of multiplications and additions to form each sample. That process requires digital circuitry which is both expensive and inflexible. Accordingly, the digital design necessary to carry out the method of harmonic summation is computationally complex and leaves much to be desired.
Another known type of musical instrument employs the filtering method of music generation. In the filtering method, a complex electrical waveform, such as a square wave or a saw-tooth pulse train, is filtered by one or more filters to select the desired frequency components. Thereafter, the filtered frequency components are combined to form the electrical signal which drives the speaker. The filtering method is commonly used to synthesize human speech and has often been used with analog electronic organs. The filtering method is comparatively inflexible since each sample relies upon the stored values of fixed samples. In order to achieve natural sound, the filtering method requires a large number of multiplication steps which are economically expensive to achieve.
In a typical example of a filter technique, a waveshape memory provides digital samples of one cycle of a waveshape to a loop circuit which includes a filter and a shift register. The digital waveshape samples read out from the waveshape memory are caused to circulate at a predetermined rate of time in the loop circuit. A output from the loop circuit varies as time lapses, and is utilized as a musical tone.
The classical filter techniques result in systems in which the pitch frequency f.sub.s /N, is determined by division using an integer, N, and hence desirable variations due to non-integral division are not achieved.
In many prior art systems, the divisor, N, is forced to be an integer when shift-register or other fixed circuits are employed. Also, the integer is further limited to some power of 2 in order to facilitate processing. In order to vary the pitch, f.sub.s /N, the frequency f.sub.s must be varied. Such systems, however, cannot be extended readily and economically to multi-voice embodiments because, for example, each voice requires a different frequency, f.sub.s.
Both the harmonic summation and the filtering methods rely upon a linear combination of sinusoids and, hence, they are characterized as linear methods for generating musical sound. The linear property is apparent from the fact that multiplying the amplitude of the input function (sinusoids for harmonic summation or a pulse train for filtering) by a factor of two results in an output waveform with the same tone quality and with an amplitude multiplied by a factor of two.
U.S. Pat. No. 4,018,121 entitled METHOD OF SYNTHESIZING A MUSICAL SOUND to Chowning describes a non-linear method for generating musical sound. That nonlinear method employs a closed-form expression (based upon frequency modulation) to represent the sum of an infinite number of sinusoids. That non-linear frequency modulation method produces a number of sinusoids which have frequencies which are the sum of the carrier frequency and integral multiples of the modulation frequency. The amplitudes of the multiples of the modulation frequency are sums of Bessel functions. The non-linear frequency modulation method of Chowning is an improvement over previously used linear harmonic summation and filtering methods, and has found commercial application in music synthesizers.
U.S. Pat. No. 4,215,617 entitled MUSICAL INSTRUMENT AND METHOD FOR GENERATING MUSICAL SOUND to Moorer describes improved non-linear methods of musical sound generation in which the amplitudes of frequency components are not constrained to the Bessel functions and in which finite spectra can be utilized, that is, spectra composed of the sum of a finite number of sinusoids.
In general, prior art methods of musical sound generation have employed deterministic techniques. Typically, the methods rely upon an input sample which has fixed parameters which specify the musical sound to be generated. Such input samples when processed by a predetermined method result in a deterministic output signal which does not have the rich, natural sound of more traditional instruments.
While many linear and non-linear methods, like those described above, have been used with success for digital musical synthesis, they all have required fast and complex computational capability typically involving several multiplication steps per sample in order to achieve rich, natural sounds. Such fast and complex computational capability results in musical instruments of high cost and complexity. This high cost and complexity has impeded the widespread availability of digital synthesis.
Accordingly, there is a need for improved musical instruments employing digital synthesis which can be used with digital circuits requiring slower and less complex computational capability than that required by prior techniques, but which still produce rich and natural sounds. There is also a need for improved digital music synthesizers which can be constructed using conventional computer processors and conventional semiconductor chip technology.
In accordance with the above background, it is an objective of the present invention to provide an improved musical instrument and method of generating rich and natural musical sounds utilizing simple and conventional digital circuitry which does not require computational complexity.